Leukemogenic Pathway of MLL-AF4 Fusion-Positive Acute Lymphoblastic Leukemia

Background

Many hematologic malignancies, including leukemia, are caused by genetic abnormalities such as chromosomal translocation. These gene defects usually perturb differentiation and proliferation of hematopoietic stem/progenitor cells. As observed in most cancers, multiple gene defects are required to develop overt leukemia, and multi-step tumorigenesis is also the rule in leukemogenesis. Leukemia-specific fusion genes are usually generated early in leukemogenesis as essential genetic defects (1^st hit) producing pre-leukemic stem cells. Additional genetic defect(s) occur in pre-leukemic stem cells, finally generating leukemic stem cells that expand to present overt leukemia.

Infant acute lymphoblastic leukemia (ALL) are frequently characterized by chromosome translocations involving 11q23, resulting in the rearrangement of the mixed-lineage leukemia (MLL) gene and subsequent generation of MLL fusion genes. Amongst the > 50 genes that have been identified as fusion partners of MLL, fusion with AF4 is characteristically and commonly observed in infant ALL, representing a hallmark of poor prognosis. In most cases of ALL with MLL-AF4, the fusion gene originates in utero as a primary event. Although mutations in RAS and FLT3 are frequently observed in MLL-AF4 -positive ALL, recent genetic data showed significantly lower number of accompanying mutations, which suggested that 2^nd hit mutations were not necessary for developing MLL fusion-positive leukemia. The necessity of 2^nd hit mutations in leukemogenesis with MLL-AF4 is still under debate.

Materials and Methods

Several previous studies have modeled leukemic transformation by introducing fusion genes into hematopoietic stem/progenitor cells; however, reproduction of certain forms of ALL, such as MLL-AF4 -positive infant ALL, has been challenging. We used the immature hematopoietic cells derived from mouse embryonic stem (ES) cells as targets for MLL-AF4 -mediated transformation.

The MLL-AF4 fusion gene was introduced into mouse ES cells to obtain ES cell clones with stable expression of MLL-AF4 . These ES cells were conditioned to generate embryoid bodies (EBs) and subsequently differentiated into hematopoietic lineages. Tie2-positive cells, which contain immature hematopoietic stem cells, were separated and transplanted into immunodeficient non-obese diabetic/Shi-scid, IL-2Rγ null (NOG) mice to investigate leukemia development.

Retroviral insertional mutagenesis was used to introduce additional genetic defect(s) as candidates for tumor-promoting 2^nd hit mutations. Purified Tie2-positive cells were transduced using the murine stem cell virus (MSCV) and transplanted into NOG mice.

Results

Mouse ES cells with stable expression of the MLL-AF4 fusion were viable and could proliferate and differentiate to produce EBs. However, EBs derived from MLL-AF4 -positive ES cells differentiated via the mesenchymal pathway rather than into the Tie2-positive hematopoietic lineage. Mice transplanted with Tie2-positive cells with MLL-AF4 expression did not develop leukemia within the observed period. However, addition of an additional genetic change by retroviral insertional mutagenesis accelerated tumorigenesis in the recipient mice. The MLL-AF4 -expressing Tie2-positive cells with insertional mutagenesis showed clonal expansion and formed mostly extra-medullary tumors with B220 expression, but did not fully develop into tumors of hematopoietic organs. Viral integration sites of these tumors were analyzed to determine additional genetic events inducing tumor formation. In the MLL-AF4 model, viral integration often caused overexpression of gene(s) located near the integration site.

Conclusion

Considering the in utero formation of MLL-AF4 harboring pre-leukemic stem cells in ALL, hematopoietic progenitor cells or immature fetal cells may be potential targets of leukemic transformation. Immature hematopoietic cells such as the Tie2-positive cells derived from ES cells could be suitable candidates for transformation with MLL-AF4 . Clonal expansion of MLL-AF4 -positive cells after introduction of retroviral insertional mutagenesis suggests the necessity of additional genetic change(s) that activate certain target gene(s) in leukemogenesis with MLL-AF4 . We are currently analyzing the roles of these genes in the leukemic transformation of MLL-AF4 -positive hematopoietic cells.